Polyimides are finding wide and increasing use in various fields of industry, especially as electrical insulation and, more recently, as structural materials. The advantages of polyimides are widely known, among them being the relatively high temperatures at which they may be used, their good electrical properties and high mechanical strength. Many suffer, however, from the disadvantage of requiring fabrication from a precursor, usually the polyamic acid, since the polyimide itself is intractable. This limits the size of articles which can be manufactured because water resulting from the conversion to the final product must be allowed to escape. A small number of specialty moldable polyimides are available, but, due to expensive starting materials and difficult processing techniques, they are quite high in price.
Because of these disadvantages, many efforts have been made to modify the polyimide structure to render it more tractable. Among these efforts are the introduction into the molecule of "hinges" which render the material more processable or the introduction of some group other than the imide, such as an ester or an amide link.
Thus, after the basic polymers disclosed in British Pat. No. 570,858 and U.S. Pat. Nos. 3,179,614, 3,179,630 to 3,179,634, it has been proposed as in U.S. Pat. No. 3,231,181 to manufacture a "malt-fabricable" polyimide derived from a diamine and bis-anhydride, each of which contains a pair of aromatic rings separated by a divalent bridging group, i.e., containing carbon, nitrogen, oxygen, silicon, phosphorus, or sulphur. In each case, a single atom bridges the rings. In U.S. Pat. No. 3,190,856 the use of benzophenonetetracarboxylic acid dianhydrides is proposed. Koton et al (J.Org.Chem., USSR 4, 754 (1968) disclose somewhat complicated routes to bis-anhydrides containing other linkages. More complex linkages between the aromatic rings carrying the anhydride moieties have been achieved by the use of trimellitic and similar anhydrides, as disclosed in U.S. Pat. No. 3,182,073, in which two molecules of the anhydride of a tricarboxylic acid react with one molecule of a difunctional amine, phenol, thiol, or compound containing a combination of two such functions.
The disclosure of the above-mentioned patents is incorporated herein by reference.
While the polyesterimides or polyamideimides produced by various of these processes are more readily manufactured and fabricated than the polyimides, the ester or similar groups introduced provide a weak point in the resistance of the molecule to hydrolysis and oxidation at elevated temperatures. These disadvantages are discussed at length in "New Linear Polymers" by Lee, Stoffey and Neville (McGraw-Hill, 1967) and the different properties of the various types of "hinge" elements are discussed in "Polyimides" by Adrova, Bessoner, Lains and Rudakov (Technomic Publishing, 1970), which are incorporated by reference to illuminate the background of the present invention. French Pat. No. 1,565,700 (CA 71, 10154g) suggests a protracted route to bis-anhydrides via Friedel-Crafts reaction of o-xylene and the mononuclear reactant terephthaloyl chloride. Kogyo Kagaku Zaashi 69, 1069 (1967) (CA 68, 69435s) is similar, save that p-bis (chloromethyl) benzene is employed. Oxidation yields the dianhydride and, in the latter case, the ketone functions.